How do you address a school senior who is older than you? You should usually call them by ‘sunbae’. But you have also probably heard Korean students address their school seniors by ‘언니’, ‘오빠’, ‘형’, or ‘누나’.
Korean students call their school seniors whom they are extremely close with 언니, 오빠, 형, or 누나. A senior you have just met or whom you are not very close to would be called 선배, but that name changes to 언니, 오빠, etc once you get know them better. This reflects the Korean mind of showing one’s intimacy for people who are not your family members by using names used within the family.
Reference: Ewha Woman University (Ewha Language Center)